public class Liczba 
{
	private int x;
	Liczba(int x)
	{
		this.x=x;      
	}
	//....
	public String convert(int base) throws LiczbaException
	{
		String result = "";
		if(base <2 || base >16) {result="NULL"; return result;}//throw new LiczbaException("Wrong base given");  }
		else
		{
		int n = this.x;
		
	    if (n == 0)
	    	return "0";
	    
	    result=Integer.toString(n, base);
	    return result;
		}
	}
}